The present invention relates to an information recording/reproducing technique for an external memory apparatus of a computer system, and to a magnetic tape subsystem employing this technique.
In, for instance, a general-purpose computer system, as an external memory apparatus for storing a large quantity of data it has been known to employ a magnetic tape subsystem constructed of a magnetic tape apparatus in which a magnetic tape is used as a recording medium and a magnetic tape control apparatus for controlling this magnetic tape apparatus.
In such a magnetic tape subsystem, the recording/reproducing operations of the information with respect to the magnetic tape are normally performed in a unit of blocks of data. When the recording/reproducing operations are executed in block, starting/stopping operations of the magnetic tape are required immediately before and immediately after the respective blocks. Since the traveling speed of the magnetic tape is not constant during the starting/stopping operations, a recording/reproducing operation is not performed during these starting/stopping operations.
As a consequence, it is necessary to provide a block interval or space between successive blocks which is not utilized for recording data thereon. Reducing such a useless block interval is important in order to increase the amount of data which can be stored on a single magnetic tape such that a magnetic tape functioning as a recording medium is effectively utilized.
On the other hand, such a conventional magnetic tape subsystem is disclosed in, for instance, JP-A-57-161956.
That is, a data buffer for temporarily storing data which are transmitted/received between a magnetic tape apparatus and a higher rank apparatus is provided in a magnetic tape control apparatus interposed between the magnetic tape apparatus and a higher rank apparatus, a plurality of data blocks which are supplied from the higher rank apparatus are stored in the data buffer, and when, for instance, the number of the data blocks stored into the data buffer exceeds a predetermined threshold value, the plurality of data blocks stored within the data buffer are combined so as to be recorded on the magnetic tape asynchronously with the operations of the higher rank apparatus.
Another technique relating to the information recording method is described in, for example, JP-A-63-197057 in which, when data having a variable length is recorded on a recording medium such as an optical disk with sectors each having a fixed length used as a recording unit, the length of the variable length data is recorded on a portion of the sector into which the head portion of the data is stored, and a specific flag is recorded on another sector into which the end portion of the data is stored.
In the first former conventional technique discussed above, although there is an advantage that the performance of the magnetic tape subsystem is improved by increasing the amount of data transmission/reception per unit time between the higher rank apparatus and subsystem, no consideration is given to the case where the size of the respective data blocks is relatively small. For instance, with respect to four different sizes of data blocks, i.e., 24K bytes, 4K bytes, 1K byte and 80 bytes, the total amounts of information which can be stored within a single magnetic tape are approximately 210M bytes, 130M bytes, 53M bytes and 5.3M bytes, respectively. There is a problem that when the sizes of the respective data blocks are small, the total amount of information stored within a single magnetic tape becomes extremely small.
Furthermore, in accordance with the second conventional technique discussed above, although there is an advantage that data having a variable length can be recorded on the recording medium in which a sector having a fixed length is used as a recording unit, there is no description of a specific arrangement and a specific apparatus when this conventional technique is applied to a recording medium such as a magnetic tape in which there is no practical limitation to, for example, a recording length of data.